I personally have seen US2FormValidator used in a couple of iPhone apps and its pretty slick. I'm not sure I personally agree with the default icon choice for the error icon, because tends to look a bit like if I touch it, it will delete/clear the field.
I think that library is a great improvement over the default of just popping an alert when something doesn't validate.
Here's some additional usability benefits:
- Validation can occur as the element is blurred without interrupting the user's flow. (Don't have to wait for submit to know what they entered is invalid).
- Errors are visually-related to the field being validated. As opposed to in an alert where it tells you the name of the field, you can actually see which field is invalid.
- User may not need the detailed message to know they did something wrong -- with a solution like this they have the option (but aren't forced) to read any detailed message before being able to correct the error themselves.
Here's a video I found demoing it in action as well.